This invention relates to ink-jet recording head apparatus, and more particularly to an ink-jet recording head apparatus of charge modulation and continuous jet type.
A typical prior art one of ink-jet recording head apparatus is disclosed in, for example, U.S. Pat. No. 3,596,275. Such an ink-jet recording head apparatus is adapted for application to industrial service of printing records on, for example, bottles of refreshing beverages, fruits, etc. and is demanded to meet the following requirements:
(1) The apparatus can print records on a variety of kinds of articles which are different from one another in the material, ground color, surface configuration, etc.
(2) The apparatus can print, for example, various kinds of characters with dots of different diameters.
(3) The apparatus can operate satisfactorily under severe environmental conditions, for example, at temperatures of from 5.degree. C. to 35.degree. C.
In view of the above requirements, the following conditions are demanded for its head:
(1) In order to print records on a variety of kinds of articles various kinds of ink are required, and arrangements are required to deal with the various kinds of ink.
(2) Because of the requirement for injection of dots of various sizes, a variety of orifices of corresponding diameters must be prepared.
(3) The head is required to deal with changes in the physical properties of ink due to temperature changes.
An ink-jet recording head is disclosed in, for example, U.S. Pat. No. 3,708,118. In the prior art head disclosed in this U.S. patent, an ink pump chamber receiving a supply of ink under pressure is shaped in the form of a pipe, and an electrostrictive element or a piezo-electric element is disposed to surround the pipe. The vibration of the electrostrictive element is transmitted to writing fluid or ink in the pipe, and the resultant resonance of the fluid is utilized to amplify the vibration amplitude thereby producing droplets of ink to be ejected from the orifice.
The prior art head is so constructed that the orifice portion is fixed to the end of the pipe, and the electrostrictive element is fixed by a binder to the pipe to form an integral combination.
There is the following relation between the length l (meters) of the ink pipe, the velocity V (meters/second) of sound propagating through ink (which will be referred to hereinafter as an acoustic velocity in ink) and the resonance frequency f (kHz): EQU l=aV/.DELTA.f(a: 1, 3, 5, . . . )
On the other hand, the diameter of the orifice is determined depending on the required diameter of dots depicting characters, and the velocity of ink flow is determined so as not to exceed the limit of laminar flow. Then, the nozzle excitation frequency suitable under the above conditions is determined.
After setting the fluid resonance frequency at the value of the nozzle excitation frequency thus determined, the kind of ink to be used is determined to determine the acoustic velocity in ink (the velocity of sound propagating through ink), so that the pipe length l can be determined according to the above equation.
The prior art head disclosed in U.S. Pat. No. 3,708,118 has been defective in the following points because the structure of the nozzle is limited by the factors above described:
(a) The length of the ink pipe must be changed depending on the kind of ink (hence, the acoustic velocity), and, therefore, the head must also be changed.
(b) The nozzle excitation frequency must be changed depending on the diameter of the orifice. Consequently, the length of the ink pipe must be changed, and the head having such a pipe length must be employed.
(c) A change in the temperature results in a corresponding change in the physical properties of ink, hence, in a corresponding change in the acoustic velocity in ink. Consequently, ink droplets would not be stably produced, when the temperature tends to change.
In such an ink-jet head of resonant type, the vibration of the electrostrictive element is transmitted indirectly to ink through the walls defining the ink pump chamber.
As another type different from such a resonant type, an ink-jet head constructed to directly transmit the vibration energy to ink is disclosed in, for example, U.S. Pat. No. 3,683,212. The ink-jet head disclosed in this U.S. patent employs a piezoelectric ceramic disc, and the vibration of the ceramic disc is imparted to ink portions in contact with the outer periphery of the ceramic disc. More precisely, the vibration of the ceramic disc is transmitted radially outward from the center of the ceramic disc, that is, in a divergent direction. Consequently, the efficiency of utilization of the vibration energy has been low, and it has been difficult to stably produce ink droplets ejected from the orifice. Further, due to the fact that ink makes direct contact with the ceramic disc at only the outer periphery of the latter, it has been difficult to increase the effective area of the disc serving to impart the vibration energy to ink.